1. Field of the Invention
The present invention relates to a resin for a toner for electrostatic image development, a master batch pigment, a toner, a developer, an image forming method, and an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses such as copiers, facsimile machines, printers, and the like have been used everywhere following the increase in the office work. An image forming method using a typical electrophotographic system in such image forming apparatuses comprises a charging process in which the surface of a latent image bearing member is uniformly charged, an exposure process in which the surface is exposed imagewise and charges in the exposed zones are removed to form an electric latent image (electrostatic image), a development process in which a fine toner particle bearing an electric charge is caused to adhere to the latent image to visualize it, a transfer process in which the obtained visible image is transferred onto a transfer material such as paper, and a fixing process in which the visible image is fixed on the transfer body by heating or pressing (usually a thermal roller is used).
Two-component developers comprising a carrier and a toner and one-component developers (magnetic toners, non-magnetic toners) containing only a toner and requiring no carrier are known as the developers for developing electrostatic images formed on the latent image bearing member.
Toners used in such image forming methods comprise a binder resin and a coloring agent as the main components and, if necessary, also contain additives such as charge control agents, offset-preventing agents, and the like. The toners are required to have various properties in the above-mentioned processes. For example, in the development process, the toner and the binder resin for the toner are required to retain the amount of electric charge suited to image forming apparatuses such as copiers, printers, without being affected by ambient conditions such as temperature, humidity, and the like Furthermore, in the fixing process employing a thermal roller fixing system, the toner is required to have good anti-offset performance, that is, show no adhesion to the thermal roll, and good fixability to paper and the like. Moreover, the toner is also required to have blocking resistance properties and show no blocking during storage inside the image forming apparatuses.
In copiers, printers, facsimile machines employing an image forming method such as electrophotography, electrostatic recording, electrostatic printing, and the like, the toner is usually fixed to paper by using a heated roller which is heated to a temperature of about 100xcx9c230xc2x0 C. In such a fixing process, fixing is most often continuously conducted on a large number of paper sheets and a toner is accumulated on the heated roller in a very small amount insufficient to affect the anti-offset performance. Because of continuous rotation and continuous supply of paper, the temperature of the heated roller increases and the toner that accumulated on the heated roller is heated. As a result, the residual monomers or residual solvent remaining in the toner are evaporated, producing offensive odor.
This offensive odor is caused by starting materials present in the toner, such as binder resins, pigments, charge control agents, additives, master batch pigments in which a resin is mixed with a pigment, and the like, or by residual monomers, low-molecular-weight components, volatile components such as residual solvents, components soluble in polar solvents or polar-solvent-soluble material, various components soluble in polar solvents which are produced during toner manufacture, and the like. Therefore, it is necessary to decrease the amount of residual monomers, residual solvents, volatile components, and components soluble in polar solvents, which are present in the starting materials, toner, and developer.
With respect to those requirements, for example, in Japanese Patent Application Laid-open No. H10-10782, the evaluation was conducted by a hardly objective method of deciding as to whether the offensive odor was felt or not, or the amount of volatile components was determined by the difference in weight before and after drying and could not be fully distinguished from the amount of water-containing components. Furthermore, in Japanese Patent Application Laid-open No H11-249334, the amount of residual monomers remaining in the toner was specified, but sufficient attention was not paid to the method for detecting the monomer components, the amount of monomer components detected by usual gas chromatography was specified, and the amount of components soluble in polar solvents, which presently pose a problem, was not filly taken into account. Japanese Patent Application Laid-open No H10-10782 is merely a description of a special suspension polymerized toner and only describes in detail a method for the manufacture thereof, providing no general description. Moreover, the object is the amount of volatile components, rather than the components soluble in polar solvents representing the present object. Japanese Patent Application Laid-open No. H11-249334 is described hereinbelow. However, it contained no general description, and the effect of the present object could not be fully demonstrated. Furthermore, all of the monomer components were controlled, rather than components soluble in polar solvents of the present object. The components soluble in polar solvents also contain dimer and trimer components other than monomers, and the amount of polar solvent insolubles such as non-polar solvents, e.g., hexane and the like, in the monomers is large. Japanese Patent Application Laid-open No. H11-249334 teaches, for example, the residual monomer can be removed by restricting the amount of residual monomer by means of temperature control or by conducting distillation after polymerization, or by using a method of adding an initiator during toner manufacture by a direct polymerization method or in the manufacture of binder resin Furthermore, when the toner is manufactured by grinding, the monomers can be removed by reducing pressure during heating and kneading of starting material with a kneader, or the residual monomers can be removed with a comparatively good efficiency by using spray drying in the manufacture of the toner. In particular, when the toner is manufactured by a suspension polymerization method, the removal can be conducted even when the toner particles are heated and dried,
Furthermore, the requirement relating to the decrease in the content of residual solvents, volatile components, and components soluble in polar solvents is important not only in terms of environmental protection, that is, from the standpoint of ecology, and human safety (carcinogenic and toxic substances), but also in terms of resolving the problem of toner quality. For example, recent research conducted by the inventors clearly showed that the components soluble in polar solvents which are present in the toner caused carrier spent, decreased the amount of electric charge of the toner, and were one of the reasons for decreased image density
On the other hand, because of properties required for binder resins for toners, that is, transparency, insulating properties, water resistance, flowability (as a powder), mechanical strength, gloss, thermoplasticity, grindability, and the like, polystyrene styrenexe2x80x94acryl copolymers, polyester resins, and epoxy resins have been usually used as the binder resins. Among them, styrene resins have been widely used because of their excellent grindability, water resistance, and flowability.
The following problem, however, was associated with toners containing styrene resins and toners containing polyester resins. Thus, when copies obtained by using such toners, were stored in document holders made of vinyl chloride resin sheets, because the image surface on the copies was allowed to be in a state of intimate contact with the vinyl chloride resin sheets, a plasticizer contained in the sheets, that is, in the vinyl chloride resin, migrated onto the fixed toner image and plasticized it, causing fusion of the image to the sheet. As a result, when the copy was separated from the sheet, the toner image was partially or entirely peeled from the copy. Moreover, the sheet was contaminated.
In order to prevent such a transition of image onto the vinyl chloride resin sheets, Japanese Patent Applications Laid-open Nos. S60-263951 and S61-24025 suggested to blend an epoxy resin which is not plasticized by plasticizers for vinyl chloride resins with a styrene resin or polyester resin as a binder resin. However, when such blended resins were used, in particular, for color toners, uncompatibility of different resins caused problems associated with anti-offset performance, curling of fixed images, glossiness (absence of gloss on color toner images creates an appearance of scanty image), coloration ability, transparency, and color forming ability. Those problems cannot be completely resolved by the conventional epoxy resins or acetylated modified epoxy resins suggested by Japanese Patent Applications Laid-open No. S61-235852
Resolution of the above-mentioned problems by using an epoxy resin alone can be considered, but such an approach leads to a new problem of reaction of the epoxy resin with amines. Epoxy resins usually have a crosslinked structure obtained by the reaction of epoxy groups and a curing agent and are used as cured resins having excellent mechanical strength and chemical resistance. Curing agents can be generally classified into amine-based agents and agents based on anhydrides of organic acids. Obviously epoxy resins suitable for toners are employed as thermoplastic resins, but amines may be present in dyes, pigments, and charge control agents kneaded together with the resin in the toner and a crosslinking reaction sometimes occur during kneading, making the composition unsuitable as a toner. Furthermore, chemical activity of epoxy groups is considered to have a toxic character, for example, to irritate the skin, and fill attention should be paid to the presence thereof Since epoxy groups also demonstrate hydrophilicity, they intensely absorb water under high-temperature and high-humidity conditions, causing decrease in the electric charge, toner deposition on the background of images, and cleaning defects. One more problem is associated with charge stability in epoxy resins.
A toner is typically composed of a binder resin, a coloring agent, and a charge control agent. Various dyes and pigments are known as colorants, some of them also have a charge control ability, and some act as both the coloring agent and the charge control agent. Toners have been widely manufactured with the above-described various compositions by using epoxy resins as binders, but problems are associated with dispersibility of dyes, pigments, and charge-control agents
Thus, kneading of a binder resin and a dye or pigment, and a charge control agent is typically conducted with a thermal roll mill in which the dye or pigment and the charge control agent are dispersed in the binder resin. A fully homogeneous dispersion is, however, difficult to obtain. If the dispersion of dye or pigment serving as a coloring agent is poor, the color forming ability is degraded and the degree of coloration is also decreased. Another problem is that if the dispersion of charge control agent is poor, the charge distribution becomes non-uniform and a variety of defects are caused, for example, charge defects, toner deposition on the background of images, scattering, insufficient ID, spread, cleaning defects, and the like.
Furthermore, Japanese Patent Application Laid-open No. S61-219051 discloses a toner using an epoxy resin ester modified with e-caprolactone, and though flowability and resistance to vinyl chloride were improved, the modification ratio was 15-90 wt. %, the softening point became too low, and gloss was too intensive
Japanese Patent Applications Laid-open No. S52-86334 disclosed a positively charged composition prepared by conducting a reaction of aliphatic primary or secondary amine with terminal epoxy groups of the already prepared epoxy resin. However, as described above, a crosslinking reaction occurs between epoxy groups and amines, sometimes making the composition unsuitable for toners. Japanese Patent Applications Laid-open No. S52-156632 disclosed a process for conducting a reaction of alcohols, phenols, Grignard reagent, sodium acetylides of organic acids, alkyl chlorides, and the like with any one or both terminal epoxy groups of epoxy resins However, when an epoxy group remains, a problem is associated, as described above, with reactivity with amines, toxicity, hydrophilicity, and the like. Moreover, hydrophilic components, components affecting the electric charge, and components affecting grinding ability in toner preparation are present in the reaction product, and good toner is not necessarily obtained.
Japanese Patent Application Laid-open H1-267560 discloses a composition prepared by reacting both terminal epoxy groups of an epoxy resin with a compound containing one active hydrogen atom, followed by esterification with monocarboxylic acids, ester derivatives thereof or lactones. Though the problem of reactivity, toxicity, and hydrophilicity of the epoxy resin was resolved, curling during fixing was not particularly improved.
Furthermore, solvents such as xylene were usually widely used during synthesis of epoxy resins or polyol resins (for example Japanese Patent Applications Laid-open No. H11-189646). However, a problem was that those solvents or unreacted residual monomers such as bisphenol A were present in rather significant amounts in the manufactured resins and large residual amounts thereof were also present in the toners using such resins.
On the other hand, in a typical method for the manufacture of toners, for example, as disclosed in Japanese Patent Applications Laid-open No. H1-304467, all starting material are mixed together, then heated, melted, and dispersed with a kneader, producing a homogeneous composition which is then cooled, ground and classified producing a toner with a volume-average particle size of about 6xcx9c10 mm.
In color toners used to form color images, various color-bearing dyes or pigments are usually dispersed in a binder resin In such case, requirements imposed on the toner used are more stringent than those in case of black images. Thus, in addition to mechanical and electrical stability against external factors such as impacts or humidity, the color toners are required to have an appropriate color appearance (degree of coloration) or light permeability (transparency) when used in overhead projectors (OHB).
Color toners using dyes as coloring agents are described, for example, in Japanese Patent Applications Laid-open Nos. S57-130043 and S57-130044. When a dye is used as a coloring agent, the image obtained has excellent transparency, the color forming ability is good, and a bright color image can be formed. However; in this case, a problem is associated with a poor light resistance of toners, which results in discoloration or color fading under the effect of direct light.
Color toners using pigments as coloring agents are described, for example, in Japanese Patent Applications Laid-open Nos. S49-46951 and S52-17023. However, though the pigment-containing color toners have excellent resistance to light, the dispersibility of pigments in the binder resin is poor, degrading degree of coloration (color forming ability) or transparency
Methods for improving dispersibility of pigments in binder resins are described in Japanese Patent Applications Laid-open Nos. S62-280755 and H2-66561. However, none of those methods provided for sufficient dispersion of pigments and the degree of coloration and transparency were poor. Furthermore methods described in Japanese Patent Applications Laid-open Nos. H9-101632, H4-39671, and H4-230770 improved the dispersion of pigments, but since all of those methods used solvents, a problem was associated with a rather large amount of residual solvent. Another problem was that using the solvents resulted in increased content of components soluble in polar solvents.
The advantage of contactless thermal fixing over that using thermal rollers is that resolution of images during development is not degraded, no paper jams are caused, and high-speed printing or simultaneous printing on both sides can be effectively conducted However, with the contactless thermal fixing, the ratio of energy dissipated into environment is higher, thermal efficiency is poor, and power consumption is high For this reason, the amount of provided heat is greater than that provided by thermal rollers, heat effect on the toner is large, and the increase in the amount of volatile components or components soluble in polar solvents has to be taken into account
Furthermore, attempts were made to develop a fixing system with a low energy consumption by using thin-wall rollers having a small thickness of roller layer in order to increase thermal efficiency of fixing rollers or by using fixing with a low surface pressure of a film or belt to improve fixing efficiency Toners that have to be suitable for such systems also should be adequate for low-temperature and low-pressure fixing, and offset-free toners are required. As a result, it was necessary to decrease the average molecular weight or to increase the amount of components with a low molecular weight in order to make the toner suitable for low-temperature fixing, which inevitably could lead to the increase in the amount of volatile components and components soluble in polar solvents that have a low molecular weight.
Further, laser printers of the above-described electrophotographic system can be used as on-demand publishing systems printing only necessary materials, at necessary time, in necessary quantities A significant advantage of such on-demand publishing systems is that the printing cost is reduced, no storage is required for printed products, and delivery period is shortened However, the amount of printing per day increases according to customers"" demand Thus, one or several laser printers are operated in a room of limited space and printed products are produced within the whole day. As a result, the consumption of toner greatly increases, and the conventional toners and developers containing a large amount of components soluble in polar solvents have been shown to constitute a threat to operator""s health.
The inventors have discovered that a toner, a binder resin, a master batch pigment, a developer, an image forming method, and an image forming apparatus producing little offensive odor, constituting no threat to operator""s health and safety, having excellent environmental charge stability and environment protection ability, causing few troubles such as image density decrease, and providing for excellent balance of fixability and resistance to offset as well as excellent color reproducibility and brightness can be obtained by decreasing the amount of components soluble in polar solvents, which are contained in the toner, binder resin, master batch pigment, developer, and the like.
The mechanism is presently not clear, but it can be suggested that the components (for example, bisphenol A and the like) soluble in polar solvents, which are present in the toner, adhere to, and are spent on the carrier surface (the surface of a development sleeve in case of a one-component development system) contributing to electric charging of the toner during stirring and charging, which reduces the charging ability of the carrier, causes improper charging of the toner, and causes the decrease in image density.
The toner for electrostatic image development in accordance with the present invention is composed of a binder resin and a colorant or coloring agent and comprises components soluble in polar solvents at 1000 xcexcg/g or less.
The resin for a toner in accordance with the present invention is suitable as a binder resin of a toner for electrostatic image development and has an epoxy resin portion and a polyalkylene group-containing portion at least in the main chain.
The method for the manufacture of a resin for a toner in accordance with the present invention is a method for the manufacture of a resin for a toner suitable as a binder resin of a toner for electrostatic image development and having an epoxy resin portion and a polyalkylene group-containing portion at least in the main chain, the method comprising at least a step of adding water at any stage from before the synthesis reaction to after thereof then bubbling the liquid containing the water under reduced pressure, and evaporating the liquid component containing the water
The master batch pigment for a toner in accordance with the present invention is a master batch pigment suitable for a toner for electrostatic image development, the master batch pigment having a pigment dispersed in a resin for a toner having an epoxy resin portion and a polyalkylene group-containing portion at least in the main chain.
The method for the manufacture of a master batch pigment in accordance with the present invention is a method for the manufacture of a master batch pigment for a toner suitable for a toner for electrostatic image development and having a pigment dispersed in a resin for a toner having an epoxy resin portion and a polyalkylene group-containing portion at least in the main chain, the pigment being a dry powder pigment, the method comprising at least the step of preparing a mixture by mixing the dry powder pigment, the resin for an electrostatic image development toner, and water, and heating and kneading the mixture to remove the water,
The electrostatic image developer in accordance with the present invention comprises at least a toner for electrostatic image development composed of at least a binder resin and a colorant and comprising components soluble in polar solvents at 1000 xcexcg/g or less.
The image forming method in accordance with the present invention comprises a latent electrostatic image formation step in which a latent electrostatic image is formed on a latent electrostatic image bearing member and a development step in which a toner image is formed by developing the latent electrostatic image with an electrostatic image developer contained in a developing apparatus, composed of at least a binder resin and a colorant, and comprising components soluble in polar solvents at 1000 xcexcg/g or less.
The image forming apparatus in accordance with the present invention comprises a latent electrostatic image bearing member, latent electrostatic image forming means for forming a latent electrostatic image on the latent electrostatic image bearing member, and developing means for forming a toner image by developing the latent electrostatic image, this developing means enclosing an electrostatic image developer composed of at least a binder resin and a colorant and comprising components soluble in polar solvents at 1000 xcexcg/g or less.